Ozone production efficiency in Savanna and forested areas during the EXPRESSO experiment

NO, NOx, NOy and O-3 have been measured during the airborne EXPRESSO experi ment, November 96, which took place near Bangui (Central Africa) at the beg inning of the dry season. This period corresponds to an intense burning act ivity. Chemical and photochemical characteristics of the planetary boundary layer, which corresponds most of the time to the monsoon layer, and the Ha rmattan layer, which is situated above the latter, have been studied for sa vanna as well as rain forest areas. These two layers are very different whe n considering the chemical composition (especially for ozone and NOz) and t he photochemical age. The relationship of photochemical ozone production ve rsus photochemical NOx oxidation products has been investigated. Results in dicate an ozone production efficiency (OPE) ranging from 6.3 to 14.8 in the planetary boundary layer. Thus, this layer is characteristic of a photoche mically young and active air mass. In this layer, the ozone potential produ ction increases with the air mass photochemical age. On the other hand, the Harmattan layer shows an OPE ranging from 4.6 to 7.4. These values are cha racteristic of photochemically well-aged air masses. In this layer, the ozo ne potential production seems to be exhausted with values around 4.0 (i.e., 4 ozone molecules produced for each NOx molecule emitted).